e are attempting to identify and characterize those factors of ickettsia rickettsii which enable the rickettsiae to invade nd grow in mammalian cells. In the past year we have found a onoclonal antibody which, in all the screening tests we have one thus far, reacts in the immunofluorescence test with irulent R. rickettsii but not with other avirulent and irulent rickettsiae of the spotted fever group. This onoclonal antibody reacted with five hemolymph-associated rganisms of the 34 microbial agents found in a survey of 310 icks collected in the Bitter Root Valley of western Montana. hese results offer hope that this monoclonal antibody may be sed to identify individuals bitten by R. ickettsii-parasitized ticks well before symptoms of the isease would be apparent. In a separate study, two ubstances, presumably located on the surface of R. rickettsii, ere found to preferentially adsorb to glutaraldehyde-fixed L ells. As yet, we have not determined whether these factors re actually involved in the infection of susceptible cells by iable rickettsiae.